Drivers of Tree Root Exudation in Forest Ecosystems: A Global Synthesis

Forest ecosystems are particularly relevant for global root exudation rates as they cover about 31% of Earth’s land area and store ~861 Pg C as live biomass, dead wood, litter, and soils—making them one of the largest terrestrial carbon reservoirs. Root exudation represents a dynamic carbon flux pathway linking plant allocation to soil microbial activity, potentially accounting for 7–14% of global gross primary productivity. However, current global estimates often aggregate diverse biomes (forests, grasslands, croplands) or rely on seedling experiments, leaving the specific environmental and biological drivers of root exudation in established forest ecosystems poorly quantified. This study presents the first meta-analysis of in-situ root exudation rates focusing specifically on forest trees, aiming to evaluate how biotic and abiotic factors jointly influence belowground carbon flux. We conducted a meta-analysis to compile a curated database of in-situ root exudation measurements. To ensure ecological relevance, we excluded greenhouse and seedling experiments, restricting the analysis to established forest stands during the growing season (April–November). The final dataset includes 248 monthly observations from 33 studies across 76 tree species. Time-series observations allow for an in-depth analysis of seasonal root exudation patterns. These observations were integrated with global databases (GRooT, FungalRoot, WorldClim, Harmonized World Soil Database) to test drivers of root exudation rates including climatic variables, soil types, mycorrhizal type and Root Economic Spectrum (RES) traits. Preliminary results from 189 growing-season observations indicate that exudation rates were low in tree species associated with ectomycorrhizas (that were potentially forming a sheath around root tips and reducing exudates transferred into soil). Evergreens had greater exudation rates than deciduous species, but climate was purely linked to exudation. Furthermore, exudation rates were only weakly aligned with the main axes of the RES (e.g., specific root length and root tissue density), suggesting that exudation rates vary largely independently of morphological conservation-acquisition trade-offs. Furthermore, our analyses highlight a critical lack of data outside the growing season, particularly in winter and early spring. In conclusion, ectomycorrhizas are major C sinks, with little carbon from exudates reaching soil in colonised roots. Root traits are overall poor predictors of exudation and we postulate that root tips should be measured preferentially, as tips are the site of exudation in tree roots. This synthesis provides a more robust framework for understanding rhizosphere carbon dynamics, which is vital for improving the representation of root-soil processes in global carbon models.